You actually have two problems. First, regexes don't work on arrays directly. Second, a substitution (s///) returns the number of substitutions made. You don't want $term = s/$full/$abb/g, because the = will assign that count to $term. You want =~ which means "make this regex act on" (sort of).

This could be easily fixed by slurping, rather than reading your IN into an array.

# instead of @myin = <IN>;
my $myin;
{
   local $/ = undef;
   $myin = <IN>; #slurp all to scalar
}

#instead of print "@myin\n"; @myin = s/$full/$abb/g;
print "$myin\n";
$myin =~ s/\b$full\b/abb/g
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I took the liberty of capturingmatching word boundaries (\b) and adding them back ($1 and $2), since without that code, you'd replace too much. For example, trying to replace all occurances of 'hello' with 'hi', you'd change 'Othello' into 'Othi'. ;-) Checking for word boundaries should help with that.