That won't work like that. To see what's happening, try

perl -MO=Deparse test.pl

on the code put in test.pl. For

#!/usr/bin/perl

use strict;
use warnings;

my @arr=('cool','guy','here');
my $str1="I am cool";
if($str1 =~ m/@arr/){
      print "Matched";
}
[download]

this will show you

$ perl -MO=Deparse test.pl
use warnings;
use strict 'refs';
my(@arr) = ('cool', 'guy', 'here');
my $str1 = 'I am cool';
if ($str1 =~ /@arr/) {
    print 'Matched';
}
test.pl syntax OK
[download]

Note that /@arr/ is taken literally - it so has to be, only scalars are interpolated inside regexes, not arrays. You might try something like

#!/usr/bin/perl

use strict;
use warnings;

my @arr=('cool','guy','here');

my $joined_rx = join( '|', map { quotemeta($_) } @arr );

my $str1="I am cool";
if ($str1 =~ m/$joined_rx/) {
      print "Matched";
}
[download]

but I'd prefere the solution with a loop/map based on the given array.

Hth.

Update: ikegami is right, of course. @ _is_ expanded in regexes too - but the extra spaces it introduces makes the resulting rx something that's far away from what was intended. My bad, sorry.