use strict;
my $str = ' From: <Name>
Sent: <Datetime>
To: <Name>
Subj Dear Sir/Madam
Hi Vaibhav,
Hope you are doing well.
http://developer.android.com/sdk/index.html
Sent from my Windows 10 phone';
my $regex = qr/^.*(From:|Sent:|Sent\s+From|To:|Subj|Dear\s+(Sir|Madam)|http).*$/mip;
my $subst = '';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html