use strict;
use warnings;

my $inData1 = <<DATA1;
G_00160 F_02571
G_00161 F_01082
G_00162 F_00034
G_00163 F_00035
G_00164 F_00036
DATA1

my $inData2 = <<DATA2;
F_00013 G_06670
F_00034 G_00162
F_00035 G_00163
F_00036 G_00164
F_00038 G_00165
DATA2
my $outfile;

open my $ur_ci, "<", \$inData1;
my %urCi = map {chomp; split} <$ur_ci>;
close $ur_ci;

my %matches;
open my $ci_ur, "<", \$inData2;
while (<$ci_ur>) {
    chomp;
    my ($ci, $ur) = split;

    $matches{$ci} = $ur if exists $urCi{$ur} && $urCi{$ur} eq $ci;
}

print "$_ => $matches{$_}\n" for sort keys %matches;