Adiing to what the others said, if $scalar does not contain a RegEx, but a string that you want to match, you might be interested in the quotemeta(...) function, which avoids, for example, $scalar = '.*'; to match the whole string and transforms it to \.\*. Just do something like

# first possibility
$scalar = quotemeta('.*');
$foo =~ s/$scalar/string/;

# second possibility
chomp($scalar = <>);
$scalar = quotemeta($scalar);
$foo =~ s/$scalar/string/;
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Please note also that a subtitution substitues a pattern by a string, and not by a pattern. (This is probably what you meant, but some people try things like s!<a>\d{3}</a>!\d{3}! and wonder why it doesn't work)
Cheers, CombatSquirrel.