use strict;
use warnings;

for( 'string1 NA12345 other stuff',
      'string2 67890 more stuff' )
{
    my @elements = split( /\s/, $_, 3 );
    print( '[', join( '][', @elements ), "]\n" );
}

__END__
[string1][NA12345][other stuff]
[string2][67890][more stuff]