Some of the flags can be moved to a non-capturing group: 
#!/usr/bin/perl
use warnings;
use strict;

my $string = 'abc';
my $pattern = 'B';
my $replacement = 'X';
my $flags = 'i';

$string =~ s/(?$flags:$pattern)/$replacement/;
print $string;  # aXc

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But you can't do that for /gore.

Update: Even string eval doesn't help, as plain interpolation of the $replacement can break if it contains a slash. 

eval "s/\$pattern/\$replacement/$flags"

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doesn't work either, as you can't put $1 into $replacement unless you always use /ee which makes it unsafe again.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Re: Regexp substitution using variables by choroba
in thread Regexp substitution using variables by MikeTaylor

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