use strict;
use warnings;

my $x=45;

# \$x prevents $x from being evaluated when we build the
# condition.
my $sCondition="\$x == 100";

# we're resetting $x now just to prove that
# $x will be evaluated later on when we eval the
# generated code

$x=100;

# <<EOF is like "..." but for long multi-line strings
# we didn't escape the $ in $sCondition
# so $sCondition will be evaluated as part of $sPerl.
# MAKE ABSOLUTELY SURE YOU TRUST THE VALUE OF $sCondition
# before you use $sPerl.  VERY BAD THINGS CAN HAPPEN IF
# untrusted strings are used in generated code.

# We don't need to declare $x (e.g. my $x) in the generated
# code because eval uses variable declarations in the
# surrounding code, e.g. the "my $x=45" above

my $sPerl = <<EOF;
   if ($sCondition) {
     print "Hello, World\n";
   } else {
     print "Goodbye, for now\n";
   }
   1;
EOF

# Note: $sPerl ends with 1; so that we are always 
# guaranteed to exit with a true value unless we
# aborted due to an error (in which case eval will
# return an undefined (i.e. false) value.

# We will use that fact to detect problems in $sPerl
# via the code pattern eval(...) or do {...}

print "Evaluating:\n$sPerl";
eval($sPerl) or do { warn "Couldn't evaluate <$sPerl>" };