The comma doesn't do what you think. Replace it with || and it matches that line. SSCCE:

#!/usr/bin/perl
use strict;
use warnings;
use autodie;

my %replacements = (
    'user.name@domain.com' => 'uname',
    'user@domain.com'      => 'user',
);

$_ = '    "user@domain.com:Calendar/BCA-513DD600-1B-6967B200"';

#if (m/:Calendar(?=\/)/, m/:Contacts(?=\/)/, m/(?<=\"\/)Users/, m/.+@.
++\"\s=/) {
if (m/:Calendar(?=\/)/ || m/:Contacts(?=\/)/ || m/(?<=\"\/)Users/ || m
+/.+@.+\"\s=/) {

    print "Matched\n";
    # then replace every occurrence as in list
    foreach my $key (sort keys %replacements) {
        s/\b$key\b/$replacements{$key}/g;
    }
}
print "$_\n";
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