use strict;
use warnings;
use 5.010;

my $pattern = 'x[ABC]';

my @strings = (
    'xA',
    'xB',
    'xC',
    'xY',
);

for (@strings) {
    if (/$pattern/) {
        say;
    }
}

--output:--
xA
xB
xC


##</code><code>##


use strict;
use warnings;
use 5.010;

my $string= 'my_stuff[1]=400';
my $pattern = 'my_stuff\[1]=';

say "They match." if $string =~ /$pattern/;

--output:--
They match.