perl -pe 'BEGIN {print "<pre>"} END {print "</pre>"}' file.old > file.
+new
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Not using a RE like you asked, but you can try something like:

use strict;
use Tie::File;

my $file = shift;

tie my @array, 'Tie::File', $file
  or die "can't open $file...\n";

unshift @array => '<pre>';
push    @array => '</pre>';

untie @array;
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This is actually not a good time for regular expressions, though you could certainly use them. Assuming the entire text is in a single string, might I recommend the concatenation operator?

If you really want to use a regular expression, a substitution using the $ and ^ metacharacters as well as the s modifier will work. Give perlre a read through.

I agree with kennethk, just do

$filecontent = "<pre>".$filecontent."</pre>";
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from your question I presume that you are talking about adding one line at the start of the file and one line at the end of file, then the above solution should work. (take care of writing $filecontent back to the file).
If the file size is huge, as per the above solution you are slurping the entire file, so be careful and choose another efficient way :)


---

Vivek
-- In accordance with the prarabdha of each, the One whose function it is to ordain makes each to act. What will not happen will never happen, whatever effort one may put forth. And what will happen will not fail to happen, however much one may seek to prevent it. This is certain. The part of wisdom therefore is to stay quiet.

my $input = 'existing.txt';
my $output = 'new.txt';
open ( my $in, '<', $input ) or die "Cannot open file $input for readi
+ng: $!\n";
open ( my $out, '>', $output ) or die "Cannot open file $output for wr
+iting: $!\n";
print $out "<pre>\n";
while ( <$in> ) {
    print $out $_;
}
print $out "\n</pre>\n";
close $out or warn "Error writing $output: $!\n";
close $in;
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Here is another way

use strict;
use warnings;
open (FILE,"+<","test");
local $/=undef;
my $a=<FILE>;
$a =~ s/^(.*)$/<pre>$1<\/pre>/gs;
seek(FILE,0,0);
print FILE $a;
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But you have to be careful when the file size is huge