The interpolation will happen with the heredoc. Or, make up your own convention, something that will not appear in the text itself, and use s///.

$variable = 'there';
$string = <<END;
Put my variable $variable and !here!
END
print $string . "\n";
$string =~ s/!here!/$variable/;
print $string . "\n";
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Output:

Put my variable there and !here!
Put my variable there and there
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