You do NOT need to use eval if you use qr//. This is important because, as other people are saying, eval poses a security risk if someone were clever enough to close your regular expression and put arbitrary commands after it. Instead, you can do something like this:

my $s='Tue Feb  8 11:11:11 2005: blah blah blah';
my $regex=qr(^$wday $mon\s+$mday \d{2}:\d{2}:\d{2} $year:);

findstring($s);

sub findstring($)
{
  my $line=shift;

  print "found\n" if $line=~/$regex/;
}
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I'm not sure off the top of my head if qr// will interpolate your variables. If it doesn't, then you just do this to build it:

# interpolate the variables (you have to define your
# variables to interpolate first so they work).
my $regex="^$wday $mon\s+$mday \d{2}:\d{2}:\d{2} $year:";

$regex=qr($regex); # compile the regular expression
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If the date you are looking for can change, you could create a simple subroutine that will take the weekday, month, year, and day parameters and return the compiled regular expression. That would be like this:

my $s='Tue Feb  8 11:11:11 2005: blah blah blah';
my $regex=createRegEx('Tue','Feb','8','2005');

findstring($s);

sub findstring($)
{
  my $line=shift;

  print "found\n" if $line=~/$regex/;
}
sub createRegEx($$$$)
{
  my ($wday,$mon,$mday,$year)=@_;

  my $regex="^$wday $mon\s+$mday \d{2}:\d{2}:\d{2} $year:";

  return(qr($regex));
}
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