This might work for you provided your files are each less than a couple of hundred MB. If that's the case it should a bit quicker than your algorithm above.

#! perl -slw
use strict;

my( $dir, $glob, $fieldsFile ) = @ARGV;

open FIELDS, '<', $fieldsFile or die $!;
chomp( my @fields = sort{ 
    length( $b ) <=> length( $a ) 
    } do{ local $/ = "\n"; <FIELDS> } 
);
close FIELDS;

my $re_fields = qr"(@{[ join'|', map{ quotemeta } @fields  ]})";

print "@$_" for map {
    open my $fh, '<:raw', $_ or warn "$_ : $!";
    do{ local $/; <$fh> } =~ $re_fields 
        ? [ $1, $_ ]
        : ()
} glob "$dir/$glob"; 

__END__
P:\test>type fields.txt
map
grep
List::Util

P:\test>mgrep . *.pl fields.txt
List::Util ./200083.pl
List::Util ./250802.pl
map ./282393.pl
map ./306836-trietest.pl
map ./308236.pl
map ./324513.pl
map ./328326.pl
map ./330590.pl
List::Util ./332124.pl
map ./333094.pl
map ./333937.pl
grep ./334482.pl
grep ./335040.pl
map ./336125.pl
List::Util ./336580.pl
...
[download]

You can also supply CON (or maybe /dev/tty on *nix?) in place of the last parameter to have it take the search terms from the keyboard:

P:\test>mgrep . *.pl CON
min
max
^Z
max ./200083.pl
max ./250802.pl
min ./333502.pl
min ./335040.pl
min ./336125.pl
...
[download]