If the text does not contain single quotes, you can just quote it:

my $x = "..multiline\ntext..";
print qx{ echo '$x' | sed 's/^/> /'};
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For more complicated strings, you can use

my $x = qq(..multiline\ntext.."');
open my $CMD, '|-', q{sed 's/^/> /'} or die $!;
print {$CMD} $x;
close $CMD;
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