# UNTESTED

$max = ( sort { $a <=> $b } map { length } @files )[0];
# length of smallest filename - I hope.

LIST:
for $len ( 1..$max) )
{
    $s = substr($file[0],0,$len);
    last LIST unless scalar grep { /^$s/ } @files == @files;
    # precedence issue here?
}

print "prefix: '",substr($file[0],1,$len-1), "'\n";