If I understand your intent, I think you'd be better off with a hash instead of an array for "partners" (and it would be worthwhile to get your indention into better shape):

my %partners;

$partners{$_} = undef for ( @partners ); # make a hash from your exist
+ing array

# (or preferably, just load your input directly into the hash, setting
+ the
# hash keys to be the strings to look for, and the values as "undef")

sub found
{
    open( FH, $data ) or die "Can't open $data: $!";
    flock( FH, 1 ) or die "Can't lock $data for reading: $!";

    while (<FH>) {
        my ( $user, $sur, $first, $second ) = split( /\t/ );
        print "$first $second $sur" if ( exists( $partners{$user} );
    }
}
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