• Enable strict, and warnings (always a good idea - and if you'd enabled them in the first place you would have realised that you do not assign a value to $from),
• Allow the user to enter multi-line messages,
• Apply a bit of consistency (ie. you use <> on one line, and <STDIN> on another),
• Simplify a few of the statements,
• Apply some sensible code indenting and a bit of whitespace, for readability.

And this gives us:

#!/usr/bin/perl
use strict;
use warnings;
use Mail::Mailer;

my $mailer = Mail::Mailer->new("sendmail");

print "\nRecipient Address:";
chomp (my @to = split /;/,<STDIN>);

print "\nFrom:";
chomp (my $from = <STDIN>);

print "\nSubject:";
chomp(my $subject = <STDIN>);

print "\nContent (END to end):";

my $content;
while (1) {
    my $line = <STDIN>;
    last if $line eq "END\n";
    $content .= $line;
}

$mailer->open( {
    From    => $from,
    To      => \@to,
    Subject => $subject
});

print $mailer $content;
close($mailer);
[download]

Of course, one other thing you should consider is some data validation, but that's left as an exercise for the reader :)

Cheers,
Darren :)