Use <<'PATTERNEND'. It doesn't allow/require escaping.

my $pat = <<'PATTERNEND';
\s
PATTERNEND
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<<PATTERNEND, aka <<"PATTERNEND" uses the same quoting rules as double-quote literals, so you'd need to esacape characters that are special in double-quote literals, which includes \.

my $pat = <<PATTERNEND;
\\s
PATTERNEND
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my $pat = <<"PATTERNEND";
\\s
PATTERNEND
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But qr// is better suited for regex patterns.

my $re = qr/
\s
/x;
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