Using qr (quote regex-wise) and allowing for the spaces in $line also works:

use 5.018;

my $line = '        [Category("notestrecord")]';
my $pattern = qr/[Category("notestrecord")]/;
 
if($line =~ /\s*$pattern/)
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Are sure that qr// in combination with use 5.018 magically adds the missing quotemeta? My perl 5.18.1 does not show that behavior:

>perl -e 'use 5.018; my $x=qr/a[bc]d/; say("|$_| matches |$x|: ",$_=~/
+$x/ ? "yes" : "no") for ( "abd","acd","abcd","a[bc]d" )'
|abd| matches |(?^u:a[bc]d)|: yes
|acd| matches |(?^u:a[bc]d)|: yes
|abcd| matches |(?^u:a[bc]d)|: no
|a[bc]d| matches |(?^u:a[bc]d)|: no
>
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Your script matches $line against zero or more whitespace characters, followed by one of the characters "()Cacdegnorsty. The C in $line matches this condition.

I can freely change the order of characters and remove duplicates in qr/[]/ without changing the script output:

#!/usr/bin/perl -w
use 5.018;

# 1126143

my $line = '        [Category("notestrecord")]';
my $pattern = qr/["()Cacdegnorsty]/;

if($line =~ /\s*$pattern/)
{
     print "Matched |$pattern| in |$line|";  # vbars to show spaces (&
+ lack of spaces)
}  else  {
     print "No match!";
}

=head OUTPUT:

Matched |(?^u:[Category("notestrecord")])| in |        [Category("note
+strecord")]|

=cut
[download]

>perl 1126159-modified.pl
Matched |(?^u:["()Cacdegnorsty])| in |        [Category("notestrecord"
+)]|
>
[download]

Alexander