use strict;
use warnings;

my $string = "aaaaaaaaa";
my $modifiers = "g";
eval "\$string =~ s/a/b/$modifiers;";
print $string;

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Update: note that only question 1 was answered because only question 1 was asked at the time of posting. OP performed a substantive silent update subsequent to the initial create.

DWIM is Perl's answer to Gödel

In reply to Re: How to apply modifiers to a substitution via variable? by GrandFather
in thread How to apply modifiers to a substitution via variable? by lokiloki

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