$pic is being incremented. This however has absolutely no effect on $key_MS which was set and evaluated when $pic was still 1. If you want to have it re-evaluated every time it is referenced you would need a closure or an eval or similar. eg. (with the loop replaced for clarity)

#!/usr/bin/env perl 
use strict;
use warnings;

my $pic = 1;
my $key_MS = sub { "R3R3_${pic}_IF"; };
print "Old pic value is " . &$key_MS . "\n";
$pic = 2;
print "New pic value is " . &$key_MS . "\n";

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In reply to Re^3: variable inside hash key by hippo
in thread variable inside hash key by Anonymous Monk

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