You need to build the array from the input data. Note that this appends the data in a __DATA__ section following the code. To use an external file instead, open it and use the file handle in place of DATA in the while loop.

Try this version:

use strict;
use warnings;

use strict;
use warnings;

my %triggers;
@triggers{qw(bodi forc twice mass acceler law)} = (1);

my @UnNum;

while (<DATA>) {
    chomp;
    next if ! /(\d+),/;
    
    push @UnNum, [$1, [split /\s+/]];
}

for (@UnNum) {
    my $count = \$_->[0];
   
    exists $triggers{$_} && ++$$count for @{$_->[1]};
}

print join "\n", map {"$_->[0]: " . join ' ', @{$_->[1]}} @UnNum;

__DATA__
81, galileo measur us newton basi law motion
45, galileo experi bodi roll slope act same forc weight effect make co
+nstantli speed up
50, show real effect forc chang speed bodi set move previous thought
45, meant bodi act forc keep move straight line same speed
[download]

Prints:

82: 81, galileo measur us newton basi law motion
47: 45, galileo experi bodi roll slope act same forc weight effect mak
+e constantli speed up
52: 50, show real effect forc chang speed bodi set move previous thoug
+ht
47: 45, meant bodi act forc keep move straight line same speed
[download]