Create a lookup table from file 2 with the keys being the part of the line excluding the last column. Then go through file 1 and check if the entry is in the lookup table:

open my $fh1, '<', 'file1.txt' or die $!;
open my $fh2, '<', 'file2.txt' or die $!;
open my $outfh, '>', "outputfile.txt" or die $!;

my %lines;
while (<$fh2>) {
    chomp;
    my ($key, $n) = /(.*)\s(\d+)$/;
    $lines{$key} = $n;
}

while (<$fh1>) {
    chomp;
    if (exists $lines{$_}) {
        print $outfh "$_ $lines{$_}\n";
    } else {
        print $outfh "$_ 0\n";
    }
}
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(this assumes your lookup keys are unique — in case the same key can occur multiple times in file 2, you'd have to think about which entry you then want to have in the output file...)