$line =~ s/\Q$var2\E/$var1/;

##</code><code>##

my $line = <<"HTML";
<a href="/{HOST}?action">123</a>:</b><br>[img-smile "58-41" ":)"]
HTML
chomp $line;

##</code><code>##

my $line = qq#<a href="/{HOST}?action">123</a>:</b><br>[img-smile "58-41" ":)"]#;