in reply to Search and replace everything except html tags

See Substitution outside of HTML TAGS for a previous answer to this question. @open = `cat temp.html`; Perl has perfectly good functions for opening and reading files that don't require you to fork another shell. 
foreach (@open) {
$_ =~ s/\n//ig;

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 You don't need the '$_ =~' here. Substitution operates on $_ by default. Also, this is an occassion where tr/// would be a better choice of operators. if ( "$_" eq "<(.*)>" ) { You need a regex here instead of stringwise equality, but even then it won't do what you expect. Also don't get into the bad habit of quoting scalars. If you want $_, just say $_, not "$_".

Here's one way to do this with regular expressions. Regexes, however, invariably fail on "real world" HTML. The prefered method is to use HTML::Parser or its derivatives. 

#!/usr/bin/perl -w
use strict;

open HTML, "temp.html" or die "Can't open file: $!\n";
{
    local $/;
    $_ = <HTML>;
}
close HTML;

tr/\n / /s;

while ( /([^<>]*)(<[^>]*>)?/g ) {
    print "TEXT: $text\n" if defined $1;
    print "HTML: $html\n" if $2;
}

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