You can read the entire file into a scalar variable like this

{
    open(FILE, "$filename") or die "Cant open $filename\n";
    local $/ = undef;
    $lines = <FILE>;
    close(FILE);
}
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Then you can just use your normal regular expression, but you'll probably want to use at least one of the following modifiers (from the perlre manpage):

m

Treat string as multiple lines. That is, change ``^'' and ``$'' from matching at only the very start or end of the string to the start or end of any line anywhere within the string,

s

Treat string as single line. That is, change ``.'' to match any character whatsoever, even a newline, which it normally would not match. The /s and /m modifiers both override the $* setting. That is, no matter what $* contains, /s without /m will force ``^'' to match only at the beginning of the string and ``$'' to match only at the end (or just before a newline at the end) of the string. Together, as /ms, they let the ``.'' match any character whatsoever, while yet allowing ``^'' and ``$'' to match, respectively, just after and just before newlines within the string.