It works fine for me: 

use strict;
use warnings;

my $s = 'a foo about foo above foo across foo after foo afterwards';
$s =~ s/\ba\b|\babout\b|\babove\b|\bacross\b|\bafter\b|\bafterwards\b/
+/g;

print "$s\n";
__END__
 foo  foo  foo  foo  foo

[download]

BTW, you can tighten that regexp without loss of generality: 

$s =~ s/\b(?:a|about|above|across|after|afterwards)\b//g;

[download]

Update: Thanks to graff for reminding me that the capture was not necessary. Added the ?: bit.

the lowliest monk

In reply to Re: removing stop words by tlm
in thread removing stop words by zulqernain

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