I've added one more step to also catch the API returning an integer. This is now my implementation:

        $from_api .= '.' unless $from_api =~ /\./;
        $from_api .= '00';
        $from_api =~ s,^(\d+)\.(\d{2}).*$,${1}${2},;
        $from_api += 0;
[download]

This catches all corner cases I can think of.

Truncation is still wrong.

use warnings;
use strict;
use POSIX qw/round/;
use Test::More tests => 2;


# somebody used a generating algorithm that used 32-bit single-precisi
+on floats, entered 1.13, but thought it was double-procision so print
+ed it into your database as %.15f; so now your api returns '1.1299999
+95231628' for a number intended to be exactly 1.13
# 1.12999999523162841796875                # exact 32-bit float repres
+entation of 1.13
sub get_from_api { '1.129999995231628' }   # sprintf '%.15f', 1.129999
+99523162841796875;


my $from_api = get_from_api();
print "string from_api = '$from_api' straight from api\n";
$from_api .= '.' unless $from_api =~ /\./;
$from_api .= '00';
$from_api =~ s,^(\d+)\.(\d{2}).*$,${1}${2},;
print "string from_api = '$from_api' after text manipulation\n";
$from_api += 0;

print "bad rounding = ", $from_api, "cents\n";
is $from_api, 113, "should be 113 cents";

#### redo, with proper rounding
$from_api = get_from_api();
print "string from_api = '$from_api' straight from api\n";
$from_api .= '.' unless $from_api =~ /\./;
$from_api .= '00';
$from_api =~ s,^(\d+)\.(\d{2})(\d*).*$,${1}${2}.${3},;
print "string from_api = '$from_api' after text manipulation\n";
$from_api = round($from_api);

print "good rounding = ", $from_api, "cents\n";
is $from_api, 113, "should be 113 cents";
[download]

No, truncation is correct in this case. It's not that the API returns exact binary float representations, it returns prices but somehow decided to use strings as a transport and return fractional cent digits which have been all zero for now but will throw an error in my script because they CANNOT match what's in my database because this in turn stores the prizes as integer cents.

So the context in this specific case calls for truncating as the rounding error seen in my original post was introduced by storing the string representation into a binary float.
I need to use the exact value as returned by the API, not rounded, if the API is off by one cent, it is considered an error.

BTW, aside from logic issues, I don't know how you came across the idea of using "," a comma as the separator?
Yes, this is "legal and allowed", but the difference between a . and a , can be hard to see.

I would use the default of "/" unless there is a reason not to.
My second choice would be vertical bar.
My 3rd choice would be curly braces - almost never.
I have not ever been tempted to use "," for the separator.

$from_api =~ s,^(\d+)\.(\d{2}).*$,${1}${2},;

$from_api =~ s/^(\d+)\.(\d{2}).*$/$1$2/;
$from_api =~ s|^(\d+)\.(\d{2}).*$|$1$2|;
$from_api =~ s{^(\d+)\.(\d{2}).*$}{$1$2};
[download]

That was just copied from parv's earlier post. While I tend to use / as a separator when writing my own regex I usually don't care too much when copying and just leave it what it was.