\s means match a single space, \s+ means match one or more spaces.

In the example below:

use strict;
use Data::Dumper;

my $string = 'a, 1, 2,  3';

my @split_comma = split /,/, $string;
print "\@split_comma =>", Dumper(\@split_comma);

my @split_comma_space = split /,\s+/, $string;
print "\@split_comma_space =>", Dumper(\@split_comma_space);
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Notice in the output -

@split_comma =>$VAR1 = [
          'a',
          ' 1',
          ' 2',
          '  3'
        ];
@split_comma_space =>$VAR1 = [
          'a',
          '1',
          '2',
          '3'
        ];
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Splitting with comma only leaves spaces in array elements, while splitting with comma+space leaves no spaces in array elements.

What split /,\s+/ does is to set the separator as comma followed by 1 or more spaces, thus eliminating the spaces from the result.