Win8 Strawberry 5.8.9.5 (32)  Thu 11/26/2020  5:05:35
C:\@Work\Perl\monks
>perl
use strict;
use warnings;

my $pattern     = '(.*)\.(.*)\.(.*)';
my $replacement = '$1-$3:$2';
my $flags       = '';

# $got_g is true if /g modifier present in flags.
# ($flags, my $got_g) = sanitize_flags_detect_g($flags);

fixup_forward_slashes($pattern, $replacement);

my $value = 'PR.123.ABC';

print "replacement '$replacement' \n";

my $eval_string = "\$value =~ s/$pattern/$replacement/$flags";
print "eval_string '$eval_string' \n";

eval $eval_string;
print "eval err '$@' \n";
print "output '$value' \n";

sub fixup_forward_slashes { s{/}'\/'g for @_; }
^Z
replacement '$1-$3:$2'
eval_string '$value =~ s/(.*)\.(.*)\.(.*)/$1-$3:$2/'
eval err ''
output 'PR-ABC:123'
[download]

And, as LanX showed in Re^3: Regexp substitution using variables, you can now try

$replacement = '@{[`ls`]};  # use dir on MSWin
[download]

or even rm -rf / or deltree or whatever the equivalent is. This is too strong.
map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

I fully agree that trying to offer the "full power" of Perl regexes (or even string interpolation) is fraught with difficulties, dangers and dangerous difficulties. But MikeTaylor seems determined to forge ahead with the confidence of a sleepwalker. So be it; he's been warned.

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Give a man a fish:  <%-{-{-{-<