If I've read the question right, a sensible way to do this is to use the i modifier, dependent upon a command-line parameter.

if ($case == 0)
{
    $foo =~ s/INSERTREGEXHERE/i;
}
else
{
    $foo =~ s/INSERTREGEXHERE/;
}
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In the case where we have stored our command line switch from @ARGV in $case as a binary flag - 0 for case-insensitive and 1 for case sensitive.

Is this what your looking for?

Elgon

UPDATE: Updated after incorrectly reading question for the first time. D'oh.

"What this book tells me is that goose-stepping morons, such as yourself, should read books instead of burning them."
       - Dr. Jones Snr, Indiana Jones and the Last Crusade