I think my $matching_key, is returning the number of keys that match the grep and not the actual hash key.

Then you should write a program to check

$ perl -le " print scalar grep /./, qw/ a b c /; "
3
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Yup, grep in scalar context returns the number of matches.

Now in list context it returns a list :)

$ perl -le " print for grep /./, qw/ a b c /; "
a
b
c
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So to get the first from the list, we use parens

$ perl -le " ( $foo ) = grep /./, qw/ a b c /;  print $foo "
a

$ perl -le " print( ( grep /./, qw/ a b c /  )[0] ); "
a
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See Tutorials: Context in Perl: Context tutorial

And see again Re: help with loop and start making functions, your program is very hard to read :)