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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In reply to Re: Reusing a complex regexp in multiple spots, escaping the regexp by ikegami
in thread Reusing a complex regexp in multiple spots, escaping the regexp by ecm

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