All worked here (FreeBSD 13, Perl 5.32, Path::Tiny 0.118) ...

# perl -Mstrict -Mwarnings -MPath::Tiny -E \
     'say join qq[\n],
            map path( $_ )->absolute(),
               q[~/tmp],
               q[~tester/tmp],
               qq[/tmp/$ENV{USER}]
     '
/home/parv/tmp
/home/tester/tmp
/tmp/parv
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Very odd. Thinking to step ouside of Linux, I tested this on Windows. It appears to have worked correctly for me as well.

SKMDSK06::T:\>perl -V | findstr uname
    uname='Win32 strawberry-perl 5.32.0.1 #1 Sun Aug  2 19:46:04 2020 
+x64'
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SKMDSK06::T:\>set user
USERDOMAIN=SKMDSK06
USERDOMAIN_ROAMINGPROFILE=SKMDSK06
USERNAME=Steve
USERPROFILE=C:\Users\Steve
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SKMDSK06::T:\>type test01.pl
use strict;

use Path::Tiny;

my $path = path("/a/b/c/$ENV{USER}")->absolute;

print "$path\n";
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SKMDSK06::T:\>perl test01.pl
T:/a/b/c

SKMDSK06::T:\>
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Not sure that this helps any, but there it is.

Perl is not bash, and has different interpolation syntax. If you want to interpolate the value of an environment variable into a string in Perl, you say $ENV{USER}, not $USER.

As to why you got a literal /a/b/c/$ENV{USER}, are you sure you used "/a/b/c/$ENV{USER}" and not '/a/b/c/$ENV{USER}'? The single quotes do not interpolate.

Because env vars are accessed using $ENV{USER}, and there presumably no var named $USER in your program.