# Untested...
use File::Basename;
use File::ReadBackwards;

tie *FH, 'ReadBackwards', 'filename' or die $!;

my $found;
while( <FH> ) {
  chomp;
  next unless basename($_) =~ m/^(\d+)$/;
  $found = $1;
  last;
}

die "No matches found.\n" unless defined $found;
print $found;