I have a hard time understanding your question. Why would you want to "extract" a known substring from another: you already have it! Maybe you meant "remove"? If that's the case, then the simplest thing IMO is to use a regular expression:

$string =~ s/\Q$substring//;
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Alternatively, you can use substr, index, and length:

my $offset = index( $string, $substring );
substr( $string, $offset, length $substring, '' ) if $offset > -1;
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