what's wrong with this?

my $first   = shift || die "first file\n";
my $sec     = shift || die "second\n";
[download]

The  || operator has higher precedence than the  = operator so you need to either enclose the assignment in parentheses or use the lower precedence  or operator.

You can accomplish the same result more simply with a single hash:

#!/usr/bin/perl
use strict;
use warnings;

my $first = shift or die "first file\n";
my $sec   = shift or die "second\n";

open my $FIRST, '<', $first or die "$first: $!";
open my $SEC,   '<', $sec   or die "$sec: $!";

print "processing $first\n";
my %common = map { /^\s*(\?.*?)\s/, 0 } <$FIRST>;

print "processing $sec\n";
/^\s*(\?.*?)\s/ and $common{ $1 }++ while <$SEC>;

print "finding common keys\n";
delete @common{ grep !$common{ $_ }, keys %common };

print "there are @{[scalar keys %common]}:\n";
system 'pause';
for my $key ( keys %common ) {
    print "\n\n\n$key\n\n\n";
    }
[download]