You don't print and Perl is warning you that it's a Useless use of string in void context giving you even the line number.

     "no matches for $cust\n";  # <--- Wot ?

edit

FWIW I would have collected all results in an array (and other changes improving readability)

use strict;
use warnings;
use autodie;

my @test_svc = (
    "test\@customer1.service", "test\@customer2.service",
    "test\@custome3.service",  "test\@customer4.service",
    "test\@customer5.service", "test\@customer6.service",
);

my @customers = qw/ customer1 customer2 customer33 customer4 customer5
+ /;

for my $cust (@customers) {
    if ( my @matches = grep { /$cust/ } @test_svc ) {
        print "$cust matches:\t", (join "\t", @matches) ,"\n";
    }
    else {
        print "no matches for $cust\n";
    }
}
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