I can't say I recommend this, but a sort of minimal change would be:

my $patternreadfromfile
    = q{<h1 tag="@{[substr "$2",1,-1]}">@{[substr "$1",1,-1]}</h1>};
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It is probably more comprehensible to do something like this:

my $patternreadfromfile= q{do_what_i_want( $1, $2 )};

sub do_what_i_want {
    my ($one, $two) = @_;
    for ( $one, $two ) {
        s/\A.(.*).\z/$1/;
    }
    return qq{<h1 tag="$one">$two</h1>};
}

my $outpattern = $patternreadfromfile;
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Of course, you'll want to replace my silly names with your own silly names. If you don't want your namespace polluted with this sub, you could make it an anonymous sub in a lexical that will go away when you're done with it. In that case, it looks like this:

my $doit = sub {
    my ($one, $two) = @_;
    for ( $one, $two ) {
        s/\A.(.*).\z/$1/;
    }
    return qq{<h1 tag="$one">$two</h1>};
};
my $patternreadfromfile= q{$doit->( $1, $2 )};
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