$ cat torres09.pl
#!/usr/bin/perl -w 
use strict;


while(my $line=<DATA>){
        chomp $line;
        if ( $line =~ m/^\d+$/){
                printf "%s\n",$line;
        }
}


exit(0);
__END__
0123
0444
A123


$ perl torres09.pl 
0123
0444