Moving the html out of your script using some templating tool is obviously the best solution.
Many have suggested the use of here-docs (<<EOF).

I dislike heredocs. They make me feel old, they're ugly (no offense).

However, I do like being able to put literal linebreaks in string constants.

my $foo = 'First line
Second line
Third line';
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But that's even worse than a here-doc. Fortunately, 90% of all cases where I want multiline string constants, it's about a piece of HTML. And HTML parsers don't really care about whitespace.

my $foo = '
    First line<br>
    Second line<br>
    Third line<br>
';
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That looks a lot better. But there's another problem:

my $foo = "
    First line<br>
    $second_line<br>
    <a href=\"foo.html\">Third line</a>
";
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Some people love backslash escapes, but I hate to escape my delimiter. But perl can handle alternative delimiters. '' is q// and "" is qq//. Instead of /, any non-whitespace character can be used (note: if the character is alphanumeric, whitespace is _required_ after the operator's letters (q mfoom eq 'foo'). You can use grouping characters (() [] {} [] <>) and perl will keep track of nesting.

my $foo = qq{
    First line<br>
    $second_line<br>
    <a href="foo.html">Third line</a>
};
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Read about heredocs, quote-like operators, backslash escapes and DWIM (Do What I Mean) in perlop.

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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