use strict;
use warnings;

for( 'string1 NA12345 other stuff',
      'string2 67890 more stuff' )
{
    if( $_ =~ m/^(\S+) ((?:NA)?\d+) (.*)/ )
    {
        print "matched: $2\n";
    }
}

__END__
matched: NA12345
matched: 67890