To just capture warnings without all of STDERR, you can use a $SIG{__WARN__} handler. Here's an example:

#!/usr/bin/perl

use warnings;
use strict;

warn "Before warn handler\n";
my $warnings;
{
  local $SIG{__WARN__} = sub { $warnings .= join("",@_) };

  my $var = "9nine";

  warn "In warn handler\n";
  print $var + 1,"\n";
  warn "Towards end";
}
warn "After warn handler\n";
print "Collected warnings:\n$warnings\n";
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