But you told it to match the string chr12! You can't place code in a string literal and expect it to be executed.

You want any of the following:

s/\014//g;            # By octal
s/\x0C//g;            # By hex
s/\x{C}//g;           # By hex (leading zeros allowed)
s/\N{U+C}//g;         # By hex (leading zeros allowed)
s/\N{FORM FEED}//g;   # By name
s/\N{FF}//g;          # By alias
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You could also use the character literally.

my $ff = chr(12);
s/\Q$ff\E//g;
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PriNet:

You can't have arbitrary code in the left of a substitution regular expression. (If you use the e modifier, though, you can do so on the right side.)

So you either need to use interpolation:

$form_feed = chr(12);
$var =~ s/$form_feed//g;
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Or you can just specify the value using a hex or octal literal \x0c or \014:

$var =~ s/\014//g;
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...roboticus

When your only tool is a hammer, all problems look like your thumb.