my $target = 'foo.txt';
    my $file = do {
        my $target = undef;
        foreach my $dir( qw{ /tmp /var/tmp /foo /bar } ) {
            my $t = "$dir/$target";
            # nb: you might want to use -e here
            # but -f might be what you *really* want, YMMV
            if( -f $t ) {
                $target = $t;
                last;
            }
        }
        $target;
    };

    if( $target ) {
        open IN, $target or die "Cannot open $target for input: $\n";
        # ...
    }
    else {
        die "Can't find $target.\n";
    }