my $head1="$...";

with

my $head1='$...';

This should work as long as there are no single quotes ' in your string.

my $head1=q{$...}; works too. For details see Quote and Quote like Operators

print "\$adollarsign";
[download]

Is surely in the docs but i cannot find it easely.. anyway look at quoted-interpolated-and-escaped-strings

PS perlop tell us Unary "\" creates a reference to whatever follows it. See perlreftut and perlref. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation.

PPS quotemeta can be also helpfull in your case. You can also use single quote or heredoc to prevent interpolation.

L*

#!usr/bin/perl
use warnings;
use strict;

#to print something verbatim, just use a single tick quote, ' versus "

my $head1='$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg
++=0 $end';
print $head1;

__END__

Prints:
$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg+=0 $end
[download]

#!usr/bin/perl
use warnings;
use strict;

# I have no idea what the input file (here __DATA__) is supposed
# to mean and be used?

print "*** no idea what this stuff means***\n";

while (<DATA>)  ##I just print this stuff....
{
  print;
}
print "********\n";
print "\n\n";

my $head1='$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg
+=0 $end';
my $head2="H  1    0.00    0.00     ";
my $head3="H  1    0.00    0.00     ";
my $head4="H  1    0.00    0.00     "; 
my $foot='$END'."\n";

for(my $R=0.05;$R<=10.50;$R=$R+0.05)
{
    my $filename=sprintf("H4_Linear_RHF_%05.2f",$R);
    printf ("%s %05.2f %s",  "Writing bond length", $R, "to file $file
+name\n");
    
    my $Rstring=sprintf("%.2f",$R);
    my $R1string=sprintf("%.2f",$R*2);
    my $R2string=sprintf("%.2f",$R*3);
    my $s= "$head1\n $head2 $Rstring\n $head3 $R1string\n $head4 $R2st
+ring\n$foot";
    print "$s";

}
=EXAMPLE PRINTOUT....

This uses the POD (Plain Old Documentation) tags to "cheat" and add
an example printout to the code.  Of course this has to be before the
__DATA__ segment.  If there was no __DATA__ segment, then a simple
"__END__" would work.

*** no idea what this stuff means***
  $contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg=0 $end
 $system memory=8000000 $end
 $SYSTEM TIMLIM=600 MEMORY=200000 $END
 $BASIS EXTFIL=.T. GBASIS=full $END
 $Data 
H4 Linear RHF/aug-ccpvtz
C1 
H  1    0.00    0.0     0.00
H  1    0.00    0.0     2.88
H  1    0.00    0.0     5.76
H  1    0.00    0.0     8.64                        
 $END
********


Writing bond length 00.05 to file H4_Linear_RHF_00.05
$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg=0 $end
 H  1    0.00    0.00      0.05
 H  1    0.00    0.00      0.10
 H  1    0.00    0.00      0.15
$END
Writing bond length 00.10 to file H4_Linear_RHF_00.10
$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg=0 $end
 H  1    0.00    0.00      0.10
 H  1    0.00    0.00      0.20
 H  1    0.00    0.00      0.30
$END
Writing bond length 00.15 to file H4_Linear_RHF_00.15
$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg=0 $end
 H  1    0.00    0.00      0.15
 H  1    0.00    0.00      0.30
 H  1    0.00    0.00      0.45
$END
Writing bond length 00.20 to file H4_Linear_RHF_00.20
$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg=0 $end
 H  1    0.00    0.00      0.20
 H  1    0.00    0.00      0.40
 H  1    0.00    0.00      0.60
$END
Writing bond length 00.25 to file H4_Linear_RHF_00.25
$contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg=0 $end
 H  1    0.00    0.00      0.25
 H  1    0.00    0.00      0.50
 H  1    0.00    0.00      0.75
$END

... many lines deleted ....

=cut

__DATA__
  $contrl scftyp=rhf RUNTYP=energy coord=unique MULT=1 icharg=0 $end
 $system memory=8000000 $end
 $SYSTEM TIMLIM=600 MEMORY=200000 $END
 $BASIS EXTFIL=.T. GBASIS=full $END
 $Data 
H4 Linear RHF/aug-ccpvtz
C1 
H  1    0.00    0.0     0.00
H  1    0.00    0.0     2.88
H  1    0.00    0.0     5.76
H  1    0.00    0.0     8.64                        
 $END
[download]

open(F,">$filename.inp");

to

open my $F, ">", "$filename.inp";

Then change all subsequent references from F to $F.

See open for more details. That won't fix your current issue, but it'll level up your code and beginner status.

See open for more details. That won't fix your current issue, but it'll level up your code and beginner status.

use autodie;

I was trying to be semantically equivalent to the OP. Sure, I could have said open(F,">$filename.inp") or die;, but my intent was not to advocate for an error handling strategy one way or another. I've also never had a need to use autodie, so it's not a go-to of mine. I know people who swear by it and those who hate it. Me, I am indifferent. And lazy.